The invention relates to a method for producing a wear-resistant diamond-like coating (DLC) as well as a part subject to wear provided with such a coating, such as a part of a textile machine.
In certain machine parts, especially in textile machinery that processes thread, these parts are subject to high surface wear, with the surface either being constantly in contact with the moving thread or frequently coming in contact therewith, or coming in contact with other hard moving parts.
Specifically, when any part subject to wear must possess sufficient elasticity as a whole, attempts have already been made in many ways to treat the surface of such parts subject to wear, to reduce the wear.
In the past, this was accomplished by surface hardening of steel materials, mechanically changing the structure in the surface area by cold rolling for example or by applying a comparatively wear-resistant coating such as hard chrome plating.
In the recent past, attempts have also been made in industrial applications and for a wide variety of applications to apply to a substrate as a carrier, usually a metal material, a thin diamond layer, or a diamond-like layer, in other words a layer consisting of polycrystalline or microcrystalline diamond or hard amorphous carbon, also referred to as DLC (diamond-like carbon) or ACH (amorphous hydrogenated carbon).
Theoretically, such diamond-like layers are suitable for preventing wear in parts that come in contact with other parts that contact them two-dimensionally, as in the case of the parts of textile machinery that are subject to wear, for example the punch needles of a warp-knitting machine.
Such layers are usually deposited chemically by the CVD method from an atmosphere of gas or plasma, with the plasma being excited by exposure to the radiation of electromagnetic waves and certain pressure and temperature parameters as well as suitable precursors being present as starting materials for the deposition atmosphere.
In this connection, the diamond-like layer itself has proven to be less of a problem than its adhesion to the substrate and a layer deposition that is as uniform as possible over the areas to be coated. In order to improve adhesion, attempts have been made in the past first to apply an adhesion promoter layer to the substrate and then to deposit the diamond-like layer on top of that.
Adhesion in particular but also the deposition rate, in other words the speed with which a given layer is built up, depends on a favorable combination of the parameters involved in the method.
Therefore, one goal according to the invention is to provide a method for producing a diamond-like coating on the substrate which, despite rapid and economical buildup of the layer, produces good adhesion of the diamond-like coating to the substrate, preferably by means of an adhesion promoting layer located in between and thus to offer uniform quality over the area subjected to wear, as well as a part subject to wear that is provided with such a coating.
The deposition atmosphere is excited and the layer-forming hydrocarbons are split usually by means of exposure to radiation at radio frequency, approximately 13.56 MHz. But irradiation by other types of radiation from the kHz range to the microwave range of several GHz, and also by using DC voltage, pulsed DC voltage, medium- or high-frequency alternating fields, is also possible.
A plasma made of hydrocarbon gases for example, with or without the addition of other gases, is excited by the alternating field between the substrate and/or the substrate carrier, on which a plurality of individual substrates is usually located, and by means of the ground electrode, usually the inside wall of the reaction chamber. As a result of the bias voltage that forms on the substrate, positive ions are accelerated out of the plasma against the substrate and strike the substrate. The ion energies and ion densities of the plasma are of great significance for the properties of the coating.
The ion energies and ion densities of the plasma depend on both the gas and its pressure and on the applied electrical power and the geometry of both the parts and of their arrangement.
Therefore it has proven to be advantageous to locate the substrate in the reaction chamber on a theoretical clamping surface on which the substrates are located in such fashion that the electrical field lines that form between the ground electrode, in other words usually the inside walls of the reaction chamber or auxiliary electrodes provided for this purpose, and the substrate, to the greatest degree possible, terminate as perpendicularly as possible and spaced apart closely and uniformly, on this clamping surface.
When the clamping surface extends through the substrate, the field lines naturally terminate on the surface of the substrate to be coated and hence in front of the clamping surface. In this case, the ends of the field lines closest to the clamping surface at the end point should be directed to the greatest degree possible as perpendicularly as possible and spaced apart uniformly and closely, against the clamping surface.
Especially when a certain area is preferably to be coated on a substrate, the substrate can be located in such fashion that the main surface to be coated is located on or parallel to the clamping surface. In this case, the clamping surface for example can be an endless clamping surface that is a continuous ring in a top view within the reaction chamber. Then however a ground electrode must also be provided in its interior.
Frequently however the case arises that a two-dimensional, preferably even a plane two-dimensional part must be coated as a substrate, not only on the two two-dimensional sides but preferably also along the narrow sides. In this connection it frequently occurs in the case of parts subject to wear such as punch needles in warp-knitting machines that the inner surfaces of holes or the inwardly directed surfaces of cavities must be coated.
In this case of course, the simultaneous coating of both opposite side surfaces is desirable, for which reason a single, usually plane, surface is selected as the clamping surface, on which the relatively thin two-dimensional substrates are located in such fashion that their two-dimensional outer sides opposite one another are located on the left and right at a short distance from the theoretical clamping surface, so that the field lines that terminate on the outer sides of the substrate are directed predominantly at right angles against the clamping surface located in between.
A plane clamping surface as a rule is the standard plane of the three-dimensional arrangement of ground electrodes. With a parallelipipedic arrangement of ground electrodes, in other words a reaction chamber in the shape of a parallelepiped, in which the inner surfaces function as ground electrodes, this is preferably in the plane of symmetry of the central plane of the reaction chamber that runs parallel between the two largest inner surfaces opposite one another.
The substrates, particularly if the narrow sides of two-dimensional substrates are also to be adequately coated, must be located relative to one another in the clamping plane so that there is a sufficient distance between the substrates in order to allow a sufficient number of field lines to strike the narrow sides of two adjacent substrates facing one another. In particular, the covering of the clamping surface by the substrates, not by the substrate holders, should be only about 5 to 30% and preferably only 10 to 25%.
As the substrate, either carbon steel is used, which possibly has been hard chrome plated on the surfaces to be coated for example, or stainless steel, which consists of an alloy that contains chromium.
Also electrically non-conductible substrate are possible.
To produce the diamond-like layer, preferably the procedure is such that the substrates are initially precleaned in a suitable fashion, in other words degreased for example, treated with an alkaline cleaner, rinsed, etc.
Then, preferably in the reaction chamber itself, the oxide layer which is usually present on the surface of the substrate is removed, preferably by bombardment with argon ions or the ions of another noble gas. In theory, this can be referred to as atomic sand blasting.
Then the corrosive gas is removed and another gaseous material that serves as the precursor for formation of the deposition atmosphere for the adhesion promoting layer now to be applied is introduced.
Since the external diamond-like layer subject to wear consists of carbon and the under surface (substrate) consists of a metal lattice, usually an Fe lattice with chromium atoms embedded, the adhesion promoting layer should include a main component which (with respect to the outer diamond-like layer) can enter into a stable bond with the carbon there and/or is readily miscible with it, in other words it has approximately the same atomic size. This is the case for silicon for example, since silicon carbide represents a stable compound that can withstand mechanical loads, plus the fact that silicon is readily miscible with carbon.
On the other side, in other words between the adhesion promoter layer and the substrate, a good miscibility of the main component of the adhesion promoter layer with the outer surface of the substrate must be possible, or a stable chemical bond must be capable of being created.
The mixing can be promoted for example by bombardment with ions from the plasma or by influencing the temperature (in case high temperatures of the substrate are possible) which promotes diffusion as it rises. Bonding to the substrate takes place primarily due to the chemical bonding of the silicon to the atoms of the substrate.
Therefore, silicon is well suited as the main component of the adhesion promoter layer so that, as the precursor for this, gases are used with a silicon content that is as high as possible, such as hexamethyldisilazane, monosilane, or disilane, etc.
After the adhesion promoting layer has reached a thickness of least 10 angstroms, up to a few xcexcm, preferably approximately 50 to 150 nm, the buildup of the adhesion promoter layer is terminated and a transition is made to building up the diamond-like layer.
For this purpose, the precursor is changed, with the precursor for the adhesion promoter layer no longer being supplied and with a new precursor being added to the reaction chamber as the basic material for the deposition atmosphere for the diamond-like layer, and the gas contained in the reaction chamber is drawn off at the same time.
This switch from one precursor to another can theoretically occur in seconds with powerful evacuation pumps, but should take place continuously over several tens of seconds to a few minutes in order to produce a mixing zone at the transition between the adhesion promoter layer and the diamond-like layer.
The DLC layer is built up to a thickness of several xcexcm, preferably approximately 1 to 10, especially 2 to 3 xcexcm. With layer thicknesses below 1 xcexcm, the layer, despite its hardness, is removed too rapidly by wear, and with layer thicknesses above 10 xcexcm, especially due to internal stresses that are too high, adhesion problems arise between the diamond-like layer and the intermediate layer and the substrate.
During deposition of the adhesion promoting layer as well as the diamond-like layer, as a rule a pressure of only 5xc3x9710xe2x88x923 mbar to 5xc3x9710xe2x88x921 mbar prevails. A pressure of 5 to 10 mbar is also possible. However, 2 to 20xc3x9710xe2x88x922 mbar, preferably approximately 5xc3x9710xe2x88x922 mbar, have proven to be the optimum range of values.
Materials containing carbon and especially hydrocarbons, especially the gases methane, butane, and hexane as well as acetylene, are used as precursors for the deposition atmosphere of the diamond-like layer.
The bias voltage varies in a range between 100 volts and 1000 volts, preferably 300 to 700 volts, preferably approximately 450 volts, but the bias voltage is not regulated directly but the power regulation of the radio frequency radiated into the chamber, which is controlled in such fashion that the desired bias voltage is achieved as a result.
In addition, an external voltage, for example a DC voltage, can be applied between the ground electrodes and the substrate, but this also involves additional problems.
Excitation takes place with a transmitting frequency of 13.56 MHz, which is authorized for the purpose in Germany by the Bundespost, but other frequencies are possible ranging from the kHz range to the microwave range as well as DC voltages and pulsed DC voltages, under which conditions other values can be selected for the bias voltage and pressure.
With the additional imposition of a magnetic field, it is also possible to work with 10xe2x88x921 mbar.
The term xe2x80x9cstainless steelxe2x80x9d is understood in the present application to apply in particular to non-rusting steels.
An embodiment according to the invention will be described in greater detail below with reference to the figures.